Antiepileptic drugs act through a variety of mechanisms to modulate neuronal hyper-excitability. Their pharmacological effects can be mediated by direct interaction with ion channels or receptors, or by alteration in the mode or level of their expression. A key objective of the present study is delineation of acute effects of specific classes of antiepileptics and steroids on hippocampal neurons studied in tissue culture and in brain slice. Somatic conductances (Na+, Ca2+, K+ and GABA-mediated C1- conductances) will be studied using patch clamp techniques on isolated CA1, CA3, and dentate neurons in culture. Intra-dendritic recordings will be obtained in a hippocampal slice preparation to study the actions of antiepileptics on modulating sustained repetitive firing of Na+-mediated dendritic action potentials, GABA-mediated dendritic inhibition and dendritic Ca2+ spikes. We will test the hypothesis that certain antiepileptics exert action on dendritic conductances which are not clearly discernable in somatic recordings. The effects of antiepileptics on the excitability of the nonmyelinated mossy fibers and Schaffer collaterals will be examined to determine if these agents can act by limiting conduction from dentate to CA3 and CA3 to CA1. Putative receptor proteins for steroids are expressed in restricted regions of hippocampus. Do appropriate steroid ligands acutely modulate voltage-gated ion channels or neurotransmitter action in neurons isolated from these regions? High resolution time-lapse video recording and laser confocal microscopy will be used to examine patterns of intracellular Ca2+ levels of hippocampal neurons in culture. Changes in [Ca2+]i will studied following exposure to the excitatory neurotransmitter glutamate. Patterns of change in [Ca2+]i will be compared between soma and dendrite from cells in different regions, and after antiepileptic and steroid application. A key question here is to determine if anti-epileptics, or steroids can effect [Ca+]i by changing intracellular release or uptake. The present studies will focus on antiepileptic drug and steroid action of specific neurons in rat hippocampus studied in both culture and slice. The long-term objective of the proposed studies is to increase our understanding of ion channel organization and Ca2+ signalling in hippocampal neurons and to define the actions of certain anti-epileptics and steroids in modulating neuronal excitability in the hippocampus.